Too Low Compression?????

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GreenPOS

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Nov 4, 2014

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#1

• Nov 4, 2014

• #1

Hey guys, I'm currently putting together a 302 to boost later down the road after I mock everything up in another car. Its a stock block, .020 over with +8cc pistons, E-cam, I've done some bowl work and got the combustion chambers to 63cc, Trick flow heads worked over real good, and a cobra intake worked over real good too.... So according to a compression ratio calculator, I'm currently at 8.24:1, is this a decent low compression motor for boost? And would this motor make decent power without boost? Or is this motor gonna be a dog on the streets without boost? Also, what kinda boost pressure could I push it to with a lower compression motor like this? Sorry if it sounds like a noobie question, but I kinda am new to boost..... Thanks in advance guys.

 

RangerJoe

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#2

• Nov 6, 2014

• #2

IMO, it will be a dog N/A. It is definetly safe for boost, bit you could be another point of compressikn without issue. Most motors built that low for boost plan on some bigger amount of boost. How much do you want to run?

Joe

 

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rbohm

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tucson,az

Nov 6, 2014

#3

• Nov 6, 2014

• #3

i agree that it will be quite sluggish on the low end, and through about 3000rpm. boost pressure would really help this engine. something on the order of 8-10psi starting to come on at about 1500 rpm would be great.

 

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GreenPOS

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#4

• Nov 6, 2014

• #4

How much boost? As much as I can get away with lol... I'd like 15-20....
I could've swore I read somewhere stick compression is like 8.78ish... My numbers not much lower then that, plus all the flow added, it shouldn't be too bad right? Idk? I'm kinda learning as I go and trying to be different I guess!?!?

 

RangerJoe

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#5

• Nov 6, 2014

• #5

It's around 9:1. Lowering compression really just allows you to run a little more boost without detonation. Alot of guys run 12-15 pounds on motors in the 9-9.5:1 range. Remember that boost is really a measure of pressure in the intake manifold. A completely stock motor may make 15 lbs with a certain pulley setup, while a motor with better intake and heads will only make 10 lbs with the same pulley setup. These are only example numbers, but hopefully you get the idea. I would run the boost needed to safely make your HP goal.

Joe

 

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rbohm

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tucson,az

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#6

• Nov 6, 2014

• #6

GreenPOS said:

How much boost? As much as I can get away with lol... I'd like 15-20....
I could've swore I read somewhere stick compression is like 8.78ish... My numbers not much lower then that, plus all the flow added, it shouldn't be too bad right? Idk? I'm kinda learning as I go and trying to be different I guess!?!?

Click to expand...

reading is fundamental.

rbohm said:

something on the order of 8-10psi starting to come on at about 1500 rpm would be great.

Click to expand...

 

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GreenPOS

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Nov 11, 2014

#7

• Nov 11, 2014

• #7

RangerJoe said:

It's around 9:1. Lowering compression really just allows you to run a little more boost without detonation. Alot of guys run 12-15 pounds on motors in the 9-9.5:1 range. Remember that boost is really a measure of pressure in the intake manifold. A completely stock motor may make 15 lbs with a certain pulley setup, while a motor with better intake and heads will only make 10 lbs with the same pulley setup. These are only example numbers, but hopefully you get the idea. I would run the boost needed to safely make your HP goal.

Joe

Click to expand...

Your right, thanks for the advice. I'm just talking crazy and having big hopes and dreams lol.... I realize 10lbs is a safe realistic number. I also realize with lower compression I can run more timimg, but im alittle unfamiliar with the benefits of more timing too. I actually refigured my number and forgot to add in my overbore of .020 so im actually at 8.5ish on the CR.... Ive also researched around and found diffent views on where the mass air should be when I do boost it, how do yall feel about before or after the turbo mass air postion?

 

84Ttop

They make new pistons every day, so why worry?

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Nov 11, 2014

#8

• Nov 11, 2014

• #8

@ 8.5 compression you will be ok. N/A you may notice a loss in low end torque but under boost you will be fine and have a safe staring point. Timing affects the overall hp out put of a motor. This only holds true until you reach the threshold of detonation. Fuel choice will largely play a role here too.
FWIW we are at 9.5:1 compression and see 30psi of boost and have zero issues while keeping the timing is conservative (in my instance) around 24 degrees. This truly is a case by case basis and every single detail of your combo needs to be spelled out to help you any farther

 

mikestang63

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Nov 11, 2014

#9

• Nov 11, 2014

• #9

remember installing aluminum heads generally lowers compression over stock e7's, which is why most guys mill the heads to raise it back up. If you are running a blower or turbo, the lower compression will just make it a little safer. If N/A or nitrous, it is going to be a dog down low.